Technical concept of patient-specific, ultrahigh molecular weight polyethylene orbital wall implant

Introduction: The authors have been using patient-specific implants since 2006 and are constantly looking for new reconstructive materials, in order to create precise implants for orbital reconstruction. Such materials should be biocompatible and stable in the human body, as well as easy to machine and form into complex 3D shapes. Biocompatible ultrahigh molecular weight polyethylene (UHMW-PE) has several unique properties including high impact strength and a low friction coefficient that result in self-lubricating and thus non-sticking surfaces after processing.

Aim: To present the concept of a patient-specific, UHMW-PE orbital wall implant.

Materials and methods: The material used to manufacture the orbital implant was UHMW-PE converted into a solid block of medical polymer from a powder material. A delayed treatment unilateral orbital fracture case was chosen for reconstruction with patient-specific orbital wall implant. On the basis of computerized tomography, a virtual model of both orbits was prepared. The injured orbit was significantly enlarged due to dislocation of its walls. The 3D model of the facial skeleton was symmetrically divided into two parts. This resulted in two models - left and right orbit, then the uninjured orbit was superimposed onto the contralateral side. As a result two surfaces were created; the outer surface (taken from the injured orbit) was used to design the outer surface of the implant, and the inner (taken from the uninjured orbit) for the inner surface. By combining both these surfaces it was possible to determine the unique shape and thickness of the UHMW-PE implant that would allow for accurate reconstruction of the orbit. Following this, the CAD model was transferred to CAM software and a numerical code for a 5-axis milling machine was generated. The manufactured implant was sterilized in gas plasma and used to reconstruct three orbital walls.

Results: The thickness of the manufactured implant ranged from 0.2 mm to 1.5 mm and was successfully inserted via transconjunctival approach. The lower, medial and lateral walls were reconstructed. The correct position of the right eyeball was re-established by moving it upward and medially, which resulted in enophthalmos and diplopia correction. The described method features several advantages: accurate reconstruction of the original shape of the orbit, precise modification of local implant thickness during design of the CAD model, structural globe support combined with a thin implant, the possibility of repairing large orbital floor defects, corrections using scissor/scalpel during surgery are relatively uncomplicated, low level of morbidity, smooth edges and gradual, controlled variations in implant thickness between different regions. Disadvantages: changes to the curvature of the implant cannot be made during surgery, implant may require fixing with screws to be stabilized during the early phase of healing, long time required to design and manufacture implants (pre-op) and also UHMW-PE implants are radiolucent and cannot be imaged using X-rays.

Conclusion: UHMW-PE appears to have numerous advantages as a material for precise reconstruction of the orbits. Such patient-specific implants are durable, can even be used to reconstruct very thin walls, do not exhibit the high degree of morbidity typical for autogenous bone grafts and result in restoration of vision function.